1. Field of the Invention
A composite solid, semi-solid, or liquid state polymer electrolyte for alkali metal or alkaline earth metal batteries, and other electrochemical devices, which contains dispersed ceramic fillers, and which may be solidified by an alkali metal or alkaline earth metal triflate salt and toughened by partial evaporation of an ether and/or ester component.
2. Description of the Prior Art
In the prior art, various polymers have been used as a component of the electrolytes of solid state alkali and alkaline earth metal batteries, and various other kinds of electrochemical devices.
Among the problems associated with many polymers is that they have inherent relatively low ionic conductivity, and may react with the component materials, which may be an alkali metal, such as lithium, or other metallic anodes to form a non-conductive boundary layer, or which have a crystalline structure that prevents the free flow of ions, and hinders rechargeability.
The prior art polymer-containing electrolytes may also exhibit poor adherence to the electrodes, do not possess sufficient strength to prevent the punching through of dendrites and consequent shorting of the battery, and lack sufficient electrochemical and temperature stability at ambient temperatures.
It is known that the introduction of inorganic fillers into a polymer matrix improves the mechanical stability, the electrochemical stability, the temperature stability range of the host polymer, and improves cycling of the device due to a reduction in interfacial contact between the alkali metal anode metal and the electrolyte.
The Hope, et al., U.S. Pat. No. 5,006,431 describes a solid state polymer electrolyte for batteries, where the electrolyte is formed by mixing an ultraviolet light or electron beam curable polymer with an electrolyte, and curing the mixture by exposure to ultraviolet light or electron beam radiation, whereby the polymer cross links and forms the electrolyte.
The Beard U.S. Pat. No. 5,147,739 describes composite anodes which contain lithium or lithium anode substrates in combination with one or more insertion compounds which consist of transition metal chalcogenides or oxides as a coating or dispersion. However, these compounds form heavy oxides which are not suitable for many applications.
The use of fillers is described in the literature in articles entitled:
Composite Polyether Based Solid Electrolytes, by W. Wieczorek, Dept. of Physics, University of Guelph, N1G 2W1 Guelph, Ontario, Canada; on leave from Dept. of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warszawa, Poland; PA1 Composite Solid Electrolyte for Lithium Cells, by Emmanuel Peled, Ganesan Nagasubranian, Gerald Halpert and Alan I. Attia of California Technical Institute, for NASA's Jet Propulsion Laboratory, Pasadena, Calif.; PA1 Dielectric Relaxation Studies in Composite Polymeric Electrolytes Based on PEO-Al.sub.2 O.sub.3 Matrix, by P. Pszczolkowski, M. Siekierski, and J. Przyluski, Division of Solid State Technology, Dept. of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warszawa, Poland; PA1 Preparation and Characterization of Nanocomposites Containing Polyethers and Layered Solids, by J. P. Lemmon, J. Wu, and M. M. Lerner, Dept. of Chemistry and Center for Advanced Materials Research, Oregon State University, Corvallis, Oreg. 97331-4003; and PA1 An Application of Random and Layered Polymer Nanocomposites in Lithium Polymer Batteries--a Review, by Wlodzimierz Krawiec and Lawrence G. Scanlon, Aero Propulsion and Power Directorate, Wright Laboratory POOS-2, Wright-Patterson Air Force Base, Ohio 45433-7251, and Emmanuel Giannelis, Dept. of Material Science and Engineering, Cornell University, Ithaca, N.Y. 14853.
While the prior art disclosures may incorporate lithium triflate, they cross link the polymer for solidification of the electrolyte, which does not occur in the present invention. In addition, the prior art devices may not contain all the required compounds, such as polyethylene oxide, as called for in the present invention and do not contain lightweight oxides such as magnesium, lithium and sodium oxides.
In another example, a liquid electrolyte has been compounded, which consisted of propylene carbonate (PC) and 1, 2 dimethoxyethane (DME) and lithium perchlorate salt. However, this liquid electrolyte mixture will not solidify when polyethylene oxide (PEO) is added.
In addition, this and like mixtures which do not contain alkali metal triflate will not solidify, if for example alkali metal salts such as lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium tetrafluoroborate, and lithium bistrifluoromethylsulfonylimide are used.
While a solid state electrolyte has been made using polyethylene oxide and lithium triflate alone, it did not possess sufficient conductivity, cyclability, or strength even when propylene carbonate was added.
It is desired to improve on the reported electrolyte compositions, and it has been discovered that the addition of lightweight inorganic fillers, such as magnesium, lithium, and sodium oxide improves certain characteristics of the electrolytes, and that the compositions do not suffer from the described prior art problems.